This invention relates to nuclear reactor operation control processes, and more particularly to a process of the type described which eliminates the hazards of causing damage to nuclear fuel.
There are many type of a nuclear reactor. For example, a boiling-water reactor includes control rods and recycling systems which are important means for controlling the reactivity of the reactor and hence to control reactor power. The control rods, each of which contains boron carbide, are inserted into and withdrawn from the core so as to thereby control reactor power. For example, a boiling-water type nuclear power plant with a power output capacity of 460 MW uses about 97 control rods. Each of the recycling systems includes recycling system pipes, a jet pump and a recirculation pump. An increase in the volume of water delivered by each recirculation pump causes an increase in the flow rate of cooling water delivered to the core through each jet pump. An increase in the flow rate of cooling water flowing through the core results in a reduction in the density of steam voids produced in the core. Because of this phenomenon, each neutron is slowed down satisfactorily and the reactivity rises, thereby causing an increase in reactor power. Conversely, if the volume of water delivered by each recirculation pump decreases, then the flow rate of cooling water flowing through the core is reduced and the density of steam voids becomes high. This causes a reduction in the reactivity and a fall in reactor power.
A conventional process for controlling operation of a nuclear reactor will be described with reference to a boiling-water reactor. Generally, in a boiling-water reactor, there are provided two recycling systems. When reactor power is to be increased to 100% level, the control rods are withdrawn from the core while the recirculation pump of each system is operated at a 20% pump revolution speed. At this time, reactor power rises along a 20% pump revolution speed line. Withdrawing of the control rods is continued until the reactor power reaches an intersection of the 20% pump revolution speed line and a control rod pattern 100% constant line. Upon the reactor power reaching the intersection, withdrawing of the control rods is interrupted and control of the reactor power is effected by means of the recycling systems. More specifically, the pump revolution speed of each recirculation pump is increased, thereby causing the reactor power to rise slowly. The rise in reactor power is effected by gradually increasing the flow rate of cooling water flowing through the core so that the rise in reactor power may take place along the control rod pattern 100% constant line and the rate of increase in a linear heat generating rate may not exceed 0.06 KW/ft.hr at any and every point in the nuclear reactor. This is because where the increase in the linear heat generating rate exceeds the aforesaid level, there will arise an increased interaction between the cladding and the fuel pellets constituting each of the nuclear fuel elements, which will cause damage to the cladding and thus the nuclear fuel elements.
The increase of the flow rate of cooling water causes the reactor power to rise to a 100% level along the control rod pattern 100% constant line. That is, the reactor power moves on the control rod pattern 100% constant line from the above-mentioned intersection to a point where the flow rate of cooling water and the reactor power is both at a 100% level. With the lapse of the operation time of the nuclear reactor, the reactivity in the core is lowered due to consumption of the fuel and the reactor power falls. To compensate for this reduction in reactor power, steps must be taken to maintain the reactor power at the 100% level. It should be noted, however, that it is not desirable to increase the flow rate of cooling water flowing through the core to a level above the 100% level because this will induce caviation in the pumps of the recycling systems. For this reason, the control rods are further withdrawn from the core to prevent the fall in reactor power. However, it has detrimental effects on the nuclear elements to control the reactor power by manipulating the control rods when the reactor power is at a high level, since there is a danger of break of the fuel.
The above discussion is also applicable to the case where reactor power is preset to a 75% level and it is increased to this level. Withdrawing of the control rods is continued until the reactor power rises along the 20% pump revolution speed line and reaches an intersection of the 20% line and a control rod pattern 75% constant line. Then, the reactor power is caused to slowly rise along the control rod pattern 75% constant line by increasing the flow rate of cooling water as mentioned above, and reaches the 75% level. There will be the same problem as the case of the preset power level of a 100% level in manipulating the control rods at a high level of the reactor power.